1. Field of the Invention
This invention relates to the field of scanner apparatus and methods; and more particularly to inspection systems that scan luggage and cargo to detect explosives or other contraband materials therein.
2. Description of the Prior Art
In recent years, the prevalence of criminal activity that entails transportation of weapons and contraband materials has been a significant public concern. It has thus become vital to develop systems for detecting the presence of these materials, both being shipped in luggage or cargo and being carried by an individual. Of particular concern is the need to detect items used as weapons by terrorists, including ordinary firearms and knives, items such as explosive or incendiary substances, and materials which present biological, chemical or radiological hazards to people and property. The detection of illicit drugs and narcotics being transported is also of concern.
The detection of contraband in the context of air and rail transportation is especially challenging, given the need to examine large numbers of people and articles of luggage and cargo within acceptable limits on throughput and intrusiveness. Although physical inspection is a widely practiced and important technique, it is slow, cumbersome, labor intensive, and dependent on the alertness and vigilance of the inspector.
Automated systems that screen for contraband have been sought for many years. Various techniques have been proposed to detect contraband objects and materials either directly or indirectly. Magnetometry is widely used, and is sometime effective in detecting metallic objects carried by persons, but is not suited for screening cargo, which legitimately may contain large amounts of metal. Nuclear techniques, including x-ray, gamma-ray, neutron activation, and nuclear magnetic resonance methods, are applicable for screening inanimate objects. In some cases, they are able to detect metallic objects, including weapons and ancillary devices such as wires, power supplies, batteries, and triggering mechanisms for explosive devices. However, there increasingly exist threats posed by explosives associated with largely non-metallic objects, which the aforementioned methods are less able to detect. The advent of modern plastic explosives presents an especially significant threat. Even a modest, readily concealable amount of these substances can cause a substantial explosion. Moreover, miscreants have become increasingly adept at disguising weapons and explosive devices as ordinary, innocuous objects. As a result, more refined indirect methods for detection of explosives are urgently sought.
The most widely deployed methods to detect bulk quantities of explosives in luggage employ x-ray examination. The methods generally fall into two categories, viz. dual energy transmission imaging and computed tomography (CT) methods. However, both have inherent problems that limit their usefulness and effectiveness.
In the dual energy transmission method, luggage typically is scanned using a collimated x-ray fan beam of broad spectral range emanating from a Bremsstrahlung source. The x-rays transmitted through the luggage are first detected by a detector that is sensitive to low energy x-rays but passes high energy x-rays. A filter usually follows and serves to attenuate any remaining low energy x-rays. A second detector detects the transmitted high energy x-rays. Thus the data are separated into two broad energy bins. From these data it is possible to obtain an average atomic number of what is being inspected, since the relative attenuation of low and high energy x-rays depends on the atomic number of the material. For example, a low atomic number object (typically an organic substance) will have a fairly flat response to the x-ray spectrum under consideration; whereas a higher atomic number object (typically inorganic/metal) object will preferentially attenuate the low energy x-rays over high energy x-rays.
However, the dual energy transmission method has significant limitations that restrict its efficacy in detecting contraband items. These limitations include: 1) the limited accuracy with which the average atomic number can be determined; 2) the similarity in average atomic number of many common explosives and ordinarily carried, benign objects; and 3) the physical juxtaposition of materials in the luggage, which consequentially permits only an overall average atomic number to be obtained. As a result, baggage scanning systems in present use give undesirably high false alarm rates when operated with detection thresholds that are sufficiently sensitive to reliably identify actual contraband. The high false alarm rate, in turn, drives a requirement for extensive hand searching of luggage. The added scrutiny subjects passengers to discomfort and inconvenience and results in frequent passenger delays and disruption of schedules of airlines and the like. The incidence of false alarms also is likely to result in complacency and inattention on the part of security personnel.
The computed tomography method is a technique akin to methods commonly used for medical imaging. The CT method comprises collection of x-ray transmission data from a large number of angles to produce data slices of the object to be imaged. The data slices are then reconstructed, usually using a computer, to create images in which overlying objects can be distinguished. In a CT baggage scanning system the differences in measured x-ray attenuation by these different objects are used to infer their respective material densities. Upon detection of objects having densities at least similar to those of known explosive materials, security personnel are alerted to the need for follow-up inspection. Although CT theoretically can separate overlapping objects and determine their densities, a large number of benign objects ordinarily transported in luggage have densities comparable to those of common explosives. As a result, CT baggage scanning in practice also suffers from an undesirable high false alarm rate, leading to the same logistical difficulties as encountered with dual energy inspection systems. Although CT systems are billed as Explosive Detection Systems (EDS), the long inspection times and need for extensive human intervention due to the high false alarm rate compromises their efficiency and effectiveness.
The use of x-ray diffraction, also known as a form of coherent x-ray scattering, has been proposed as an alternative approach to contraband detection. Constructive interference of scattered x-rays occurs when the x-rays emerge from a target at the same angle and are in phase. This occurs when the phase lag between rays in a wave front is an integral number of wavelengths of the x-ray radiation. The condition of an integral number of wavelengths is satisfied for x-rays of wavelength λ scattered at an angle θ from the incident beam direction from a sample having a crystal lattice spacing d, in accordance with the following formula,λ=2d sin(θ/2).  (1)This equation is often known as Bragg's Law.
Most solid materials that are found in nature or are manufactured exist in polycrystalline form. That is, they comprise a large number of tiny, individual grains or crystallites. The atoms within each crystallite are located in regularly spaced positions, which are uniquely characteristic of a given material. These regularly spaced atoms, in turn, define a plurality of crystal lattice spacings uniquely associated with that material. As a result, the set of lattice spacings can serve as a unique fingerprint for that material. Such fingerprints may readily be determined for most ceramics, polymers, explosives, and metals.
The x-ray diffraction or coherent scattering technique is widely practiced for laboratory analysis for identifying unknown materials in a sample. Laboratory x-ray diffraction is most commonly implemented in an angular-resolved form. In its usual form, angular-resolved coherent scattering (AR-CS) comprises illumination of a sample with a narrowly collimated, line or pencil beam of monochromatic x-rays, i.e. x-rays having a wavelength λ within a very narrow range. Some of the x-ray flux incident on a sample is coherently scattered in accordance with Bragg's Law. The scattering for a polycrystalline sample comprising an assemblage of a large number of randomly oriented crystallites is concentrated in a series of circular cones, each cone having an apex at the sample and being centered on the incident beam direction and having a half-opening angle of θ. The intensity of this scattered radiation is determined experimentally at a series of values of angle θ. A graph of the scattered intensity versus angle is commonly termed an x-ray diffraction or scattering pattern, and is characterized by a plurality of narrow peaks seen at angles θi for a series of i values. The measured values of θi in turn allow corresponding values of di, the crystal lattice spacing which gives rise to the i-th diffraction peak, to be calculated from Equation (1). Unknown samples are identified by comparing the set of experimentally-determined lattice spacings di with the spacings of known materials. An unknown sample can be conclusively identified if its observed d-spacings match the d-spacings of a known sample with sufficient accuracy.
Less commonly, laboratory diffraction is implemented in an energy-resolved form (ER-CS), in which the sample is illuminated by polychromatic x-ray radiation. A polychromatic source, is one which emits radiation having a spread of energies and wavelengths, in contrast to a monochromatic source, which emits radiation having only a single wavelength and energy. It is known that the wavelength λ and the energy E of x-ray photons are connected by the equation:E=hc/λ,  (2)wherein h is Planck's constant and c is the speed of light. Bragg's law may thus be rewritten in a form more appropriate for ER-CS:(1/2d)=(E/hc)sin(θ/2)  (3)wherein χ=(E/hc)sin(θ/2) is a quantity conventionally termed momentum transfer. The ER-CS method normally employs an x-ray detector capable of resolving the energy of radiation incident thereon. The detector is positioned at a fixed scattering angle θ and detects coherently scattered radiation of a range of energies. Bragg's Law is satisfied for certain energies and d-spacings, so the detected radiation spectrum has peaks at these energies.
A material's x-ray diffraction pattern stems directly from its characteristic atomic structure and can thus serve as a unique fingerprint for identifying the material. Therefore, diffraction methods theoretically provide better discrimination and a dramatically lower false alarm rate than either CT or dual energy transmission screening methods. While x-ray diffraction in both forms is routinely practiced as a laboratory analysis method, known systems are complex and require skilled operators to collect and interpret the data. Moreover, the laboratory systems are incapable to carrying out analysis with the speed and reliability required for any practical baggage screening system. Accordingly, x-ray diffraction systems have not received widespread acceptance for baggage screening.
Both angular resolved coherent scatter (AR-CS) and energy resolved coherent scatter (ER-CS) systems have been proposed for baggage screening. Each has advantages and disadvantages. The AR-CS method can be implemented with a relatively simple x-ray detector, instead of a relatively complex and expensive energy-resolved detector. However, the AR-CS method requires a monochromatic x-ray beam, obtained either from the fluorescence of the source anode (most often made of tungsten) or by filtering a polychromatic beam. In either case, the number of x-ray photons available to scatter from the baggage for content analysis is severely limited. Typically, a filter is used to select x-ray photons having energy within a narrow range, e.g. an energy range encompassing the tungsten fluorescence lines near 59 keV. Unfortunately available filters are not perfect and reduce the number of photons of the desired energy. They also transmit extraneous x-rays having energies outside the desired range that muddle the resulting angular spectrum. The net effect of filtering the primary x-ray beam is to increase substantially the time needed to scan the baggage, since most of the x-rays emitted by the x-ray source are attenuated before reaching the luggage and so are not used.
On the other hand, the ER-CS method does not require a filter, and the entire x-ray spectrum potentially can be utilized. The disadvantage is that the detectors must be energy resolving, which makes the detector system more complex and costly. In addition the detector is usually positioned at a fixed angle. Bragg's Law generally is not satisfied for any x-rays in the most intense part of the spectrum, i.e. for energies near the 59 keV peak in the x-ray spectrum flux. As a result, only a small portion of the entire x-ray flux that impinges on the baggage is effectively used, likewise lengthening inspection times.
In particular, the x-ray flux spectrum typically emitted by an x-ray tube having a tungsten anode target and a 2 mm window is depicted by FIG. 10. Some 23% of the flux is contained within the tungsten fluorescence peaks near 59 keV. Additional fluorescence peaks at about 67 keV comprise about 7% of the total flux. At best, AR-CS methods rely on a small minority of the total flux, typically less than even the 23% in the 59 keV peaks. While the ER-CS method utilizes somewhat more of the x-ray flux, a large part of the intensity still cannot effectively use all the flux as a result of limitations inherent in the use of fixed scattering angles.
Methods that combine energy and angular resolution have also been proposed. However, these methods have generally entailed use of a highly collimated, pencil beam of x-rays. While such methods are suggested to be useful in locating contraband within an item, the tight collimation significantly limits the x-ray flux in both intensity and spatial extent, thus slowing the scanning to an undesirable degree.
Previous x-ray methods have also suffered from limitations that result from the techniques used to correct for the non-coherent absorption of x-ray flux traversing the item being interrogated. For example, some systems have employed a sidescattering technique that entails the complexity and expense of an additional detection system.
X-ray scattering methods that efficiently use x-ray flux from a source, while minimizing the exposure of baggage to radiation that is ineffectual in substance identification, are thus highly sought. Desirable methods also afford rapid and sensitive scanning for reliable identification of targeted substances without generation of unwarranted false alarms.
Notwithstanding the aforementioned approaches, there remains a need in the art for systems capable of reliably, accurately, and rapidly detecting the presence of contraband substances, especially explosives, accelerants, and illicit drugs. More particularly, there is need for systems that are readily automated for semi-continuous or continuous inspection and detection of the presence of such materials in luggage, cargo, vehicles, freight containers, and related items. Such systems are highly sought, especially in the context of airport screening, but would be equally valuable for courthouses, stadiums, schools, government offices, military installations, correctional institutions, and other public venues that might be targets of terrorist or similar criminal activity.